Unit 7

ECM

fills spaces between cells and binds tissues together; most abundant in connective tissue; composed of fibrous proteins (collagen, elastin), gel-like polysaccharides (GAGs), and adhesion proteins (fibronectin, laminin)

Tendon

rich in fibrous proteins (strength)

Cartilage

rich in polysaccharides (resists compression)

Bone

mineralized with calcium phosphate crystals

Basal lamina

thin, sheetlike support beneath epithelia

Collagen

most abundant structural protein in animals; forms triple helices, glycine allows tight packing, proline and hydroxyproline stabilize the helix, hydroxylysine forms crosslinks

Type I collagen

forms fibrils in connective tissues (tendon, skin, bone); densely packed

Type II

found in elastic cartilage, provides joint support

Type lll

found in muscles, arteries, and organs

Type IV collagen

forms networks in basal laminae; found in layers of skin

Type V

found in cornea, skin, hair, and placenta

Elastin

another ECM protein providing elasticity

Cross-linked lysine residues

stretch and recoil

Glycosylaminoglycans (GAGs)

polysaccharides of repeating disaccharides; highly negatively charged, binding cations and water to form hydrated gels; provide compression resistance and mechanical support; most linked to core proteins (proteoglycans); ex

Fibronectins

major adhesion protein of connective tissues; dimeric glycoprotein joined by disulfides; has binding sites for collagen, proteoglycans, and integrins

Laminins

major adhesion proteins of basal laminae; cross-shaped heterotrimers; bind to integrins, proteoglycans, and type IV collagen to organize the basal laminae network and link it to epithelial cells

Proteoglycan

up to 100 polymers can attach to a single core protein; made in lumen of golgi; negative charge attracts Na+ which brings in H20

Matrix metallo-proteinases

secreted enzymes that digest ECM

Integrins

heterodimers of two transmembrane subunits, designated alpha and beta; cell surface receptors that bind ECM on ECD and signaling and cytoskeletal proteins on cytosolic domain; cell surface signaling receptors that connect the extracellular matrix to the cytoskeleton.

Focal adhesions

cell-ECM junctions; built around integrins that link the ECM to actin filaments inside the cell; provide mechanical anchorage between cytoskeleton and ECM; allows cells to spread, migrate, and change shape; transmit mechanical and chemical signals between ECM and cytoplasm; regulate cell growth, survival, and differentiation

Extracellular Domain

integrins bind fibronectin, collagen, or laminin in the ECM

Transmembrane Domain

integrins span the plasma membrane as alpha/ beta heterodimers

Cytoplasmic Domain

beta-integrin tails connect to actin filaments through adaptor proteins

Cadherins

calcium-dependent cell adhesion molecules that mediate stable cell-cell adhesion; essential for maintaining tissue structure and integrity; establish stable cell-cell junctions in tissues; link cytoskeletons of neighboring cells for coordinated strength

Hemidesmosomes

integrins bound to basal lamina are anchored to intermediate filaments

Selectins

bind to carbohydrates

Junctional Complexes

include tight junctions, adherens junctions, and desmosomes

Adherens Junctions

cell-cell junctions formed by cadherins that link the actin cytoskeletons of neighboring cells; do not restrict fluid or membrane flow; coordinate cell shape changes; transmit mechanical tension between cells through actin cytoskeleton; work with tight junctions and desmosomes to form junctional complex in epithelial layers

Desmosomes

strong, spot-like cell-cell junctions that anchor intermediate filaments between adjacent cells; provide mechanical strength and stability (esp in tissues under stress); anchor keratin intermediate filaments across cells, creating a continuous structural network; provides tissue resilience; maintains tissue integrity; contribute to cell signaling and differentiation (epithelial and cardiac)

Tight junctions

specialized cell-cell junctions that form a seal between adjacent epithelial cells; creates a selective barrier that controls the movement of molecules through the space between cells; composed of transmembrane proteins; cytoplasmic tails bind to actin filaments via adaptor proteins anchoring the complex to the cytoskeleton; prevents leakage; maintains cell polarity by separating apical and basolateral membrane domains

Claudins and Occludins

form continuous strands encircling the cell; interact with counterparts on adjacent cells to seal the intercellular space

Gap junctions

specialized cell-cell communication junctions that form direct channels between the cytoplasms of adjacent cells; regulated channels with open and closed conformations; allow ions, metabolites, and small signaling molecules to pass freely; two connexons align, one on each cell; enable electrical and metabolic coupling between cells; facilitate intercellular signaling by sharing second messengers

Connexon

assembly of six connexins

Steroid hormones

hydrophobic signaling molecules derived from cholesterol; diffuse through PM and bind to intracellular receptors; these receptors belong to nuclear receptor superfamily and function as transcription factors

Histone Acetyltransferase (HAT)

adds acetyl groups to histone tails; neutralizes positive charges on histones, loosening chromatin; promotes transcription activation by allowing RNA pol to access DNA

Histone Deacetylase (HDAC)

removes acetyl groups from histones; restores positive charges, causing chromatin to condense; leads to transcriptional repression and gene silencing

Neurotransmission

the process by which neurons communicate with other neurons, muscles, or glands through chemical or electrical signals; allows nervous system to coordinate; neurotransmitters bind to their receptors on the cell surface of target cell (ligand-gated)

Action potential arrival

electrical signal travels down axon to axon terminal

Calcium influx

voltage gated ca channels open, allowing ca into the terminal

Vesicle fusion

ca triggers synaptic vesicles to fuse with the presynaptic membrane

Neurotransmitter release

neurotransmitters are exocytosed into synaptic cleft

Receptor binding

neurotransmitters bind to receptors on postsynaptic cell, causing ion flow or activation of signaling cascades

Signal termination

neurotransmitters are degraded, reuptaken, or diffused away to end the signal

Nuclear receptors

directly regulate gene transcription

Direct cell-to-cell (or cell ECM) signaling

form of cell communication where signals are transmitted through direct physical contact between cells or between a cell and the ECM; enables precise, localized control of cell behavior without secretion of soluble signaling molecules; involves membrane-bound signaling molecules on one cell interacting with receptors on adjacent cell and involves integrins binding to ECM proteins; triggers intracellular signaling cascade that affects cell shape, migration, survival, and differentiation

Endocrine

a form of long distance cell communication where hormones are secreted into the bloodstream by endocrine glands; hormones bind to specific receptors on target cells, triggering an intracellular signaling cascade; results in changes in gene expression, metabolism, or cell activity; provides slow but long-lasting effects

Paracrine

local cell communication where a cell secretes signaling molecules that act on neighboring target cells within the same tissue; signals don’t enter bloodstream; signaling cells release local mediators that diffuse through EC fluid to nearby target cells and bind to the specific receptors; this triggers intracellular signaling cascades that alter gene expression or cellular activity; rapidly degraded to limit range and duration of signaling

Autocrine

a type of cell communication in which a cell produces signaling molecules that bind to receptors on its own surface or on identical neighboring cells; cell acts as both signal sender and receiver, allowing for self regulation; ligand binds to receptor on same cell or nearby identical cells activating intracellular signaling pathways

Insulin

peptide hormone secreted by pancreatic beta cells; acts via endocrine signaling by traveling through the blood to target tissues; binds to cell surface receptor tyrosine kinase (RTK) to activate intracellular signaling cascades; promotes glucose uptake in muscle and adipose cells; stimulates glycogen, lipid, and protein synthesis and lowers blood glucose levels